Process for isomerizing glucose to fructose

ABSTRACT

PROCESS OF ENZYMATICALLY CONVERTING GLUCOSE TO FRUCTOSE WHEREIN A GLUCOSE-CONTAINING SOLUTION IS PASSED, UNDER SPECIFIC CONDITIONS, THROUGH A BED OF CELLS OF MICROORGANISMS CONTAINING CELL BOUND GLUCOSE ISOMERASE, SAID BED HAS A DEPTH TO WIDTH RATIO OF LESS THAN ABOUT 2.

United States Patent D Ser. No. 88,187

Int. Cl. ClZk 9/00 US. Cl. 195-31 F 14 Claims ABSTRACT OF THE DISCLOSURE Process of enzymatically converting glucose to fructose wherein a glucose-containing solution is passed, under specific conditions, through a bed of cells of microorganisms containing cell bound glucose isomerase, said bed has a depth to width ratio of less than about 2.

THE INVENTION This application is a continuation in part of application Ser. No. 55,996 filed July 17, 1970 now abandoned.

This invention relates to a process of enzymatically converting a portion of the glucose in a glucose-containing solution to fructose.

There are many processes known in the art for producing fructose-containing solutions. These processes may be grouped into three broad categories.

In the first category, sucrose is inverted to glucose and fructose by the use of an acid or invertase.

In the second category, glucose is converted to fructose by the use of alkaline catalysts. There are many papers and patents which disclose various alkaline catalysts and the use thereof for converting glucose to fructose. Exemplary of processes using alkaline catalysts are those disclosed, for instance, in US. Pat. 2,487,121 to Fetzer et al.; US. Pat. 2,746,889 to Langlois et al.; US. Pat. 2,354,664 to Cantor et al.; US. Pat. 3,285,776 to Scallet et al.; US. Patent 3,383,245 to Scallet et al.; and U.S. Pat. 3,305,395 to Scallet et al.

However, there are a number of distinct disadvantages associated with alkaline isomerization. For instance, due to the non-selectivity of alkaline catalysts various objectionable by-products are formed, such as large amounts of colored bodies and acidic materials. To refine alkalineisomerized liquors to remove the objectionable by-prod ucts to produce an acceptable product requires rather complicated and costly procedures.

The third category for producing fructose-containing solutions involves enzymatically converting glucose in a glucose-containing solution, e.g., corn syrup, to fructose. Various microorganisms are known in the art which produce glucose isomerase. For example, in an article appearing in Science, vol. 125, pp. 648-9 (1957), it is disclosed that an enzyme derived from Pseudomonas hydrophila will isomerize glucose to fructose. Also, British Patent 1,103,394 and Japanese Patent 7,428 (1966) to Takasaki et al. disclose that microorganisms classified as belonging to the .St reptmyces genus, such as Streptomyces flaivovirens, Strepromyces achromogenes, Streptomyces echinatus and Streptomyces albus, produce glucose 3,694,314 Patented Sept. 26, 1972 isomerase. There are many other microorganisms which are disclosed in the art as producing glucose isomerase. A few of these other microorganisms disclosed are, for instance, Aerobacter cloacae, Bacillus megaterium, Acetobacter suboxydans, Acetobacter melanogenus, Acetobacter roseus, Acetobacter oxydans, Bacillus fructose and Lactobacillus fermenti.

Because of the economics involved in producing glucose isomerase, it is of the utmost importance to use the isomerase under conditions whereby maximum yields of fructose are produced using minimum quantities of glucose isomerase. Moreover, the conditions for isomerization should be such that minimal quantities of objectionable by-products are produced.

Glucose isomerase is produced primarily intracellularly by the above microorganisms. Thus, the major portion of the glucose isomerase is found within and/or on the cell walls of the microorganisms. Normally, when these cells are used to isomerize glucose to fructose, the isomerase during isomerization is released or in extracted therefrom. When the isomerase is released or extracted from the cells it is essentially solubilized, It would involve a rather costly and complicated procedure to recover the solubilized isomerase so that it can be used in another isomerization reaction.

In an article entitled, Streptomyces Glucose-Isomerase which appeared in Fermentation Advances (1969'), pp. 561589, glucose isomerase derived from a microorganism belonging to the Streptomyces genus was fixed or stabilized on and/or within the cells. This fixation or stabilizing treatment consisted of heating cells containing intracellular glucose isomerase. This fixed enzyme was placed in a column and a glucose solution was continuously passed therethrough. As the glucose contacted the fixed enzyme, fructose was continuously produced. Although the process disclosed in this article apparently presents a partial solution to the problem of reusing glucose isomerase, it was conducted on such a small scale that the processing parameter used would not be applicable to commercial practice. It would therefore be ditficult, if not impossible, to reduce this process to commercial scale and produce a glucose-fructose-containing solution of acceptable quality.

Therefore, it is the principal object of the present invention to provide a continuous, commercial process for enzymatically isomerizing glucose to fructose.

This object and other objects may be attained in accordance with the present invention by forming a glucosecontaining solution having a viscosity of from about 0.5 centipoise to about 100 centipoise, a pH in the range of from about 6 to about 9 and containing from about 5 to about 'percent dry substance glucose, heating said solution to a temperature in the range of from about 20 to about 80 C. and passing the solution through a bed containing cells of microorganisms which have been treated to inhibit the extraction of the isomerase from the cells, having a glucose isomerase activity of at least 3 IGIU per cubic centimeter of bed and a stability value of at least 50 hours, thereby converting up to 54 percent of the glucose to fructose. The ratio of the depth to the width (or diameter) of the bed should be less than about 2. The flow rate of the glucose-containing solution passing through the bed should be such that the color of the solution exiting the bed is increased by less than 2 color units and there is no substantial production of psicose.

Various terms and expressions used in the foregoing and in the discussion and example hereinafter are defined as follows:

STABILITY VALUE The stability value is determined by placing a sufiicient amount of the fixed glucose isomerase in a column to obtain from 1000 to 4000 IGIU therein. A solution that is 3 molar in glucose at a pH of 7, 0.001 molar in CoCl and 0.005 molar in MgSO is passed through the column at a rate of from to about 80 mL/hr. The column is maintained at a temperature of 60 C. The fraction of glucose converted to fructose in the effluent is determined after 20 hours to insure that the bed of fixed glucose isomerase is under equilibrium conditions. The activity index of the fixed glucose isomerase is calculated using the following formula:

Activity Index: (R/ E 10g (0504/ (0504-1) Color was determined spectrophotometrically by measuring the absorbance at 450 m and 600 111;; of an appropriately diluted liquor in a l-cm. cell versus water as a reference. The spectrophotometer was a Beckman DK- 2A, manufactured by Beckman Instrument Company. The color was calculated by using the following formula:

( 45o sool) C Color units:

A =absorbance at 450 m A =absorbance at 600 mp C=concentration in grams of dry substance per 100 ml.

of liquor.

FRUCTOSE CONTENT OF ISOMERIZED LIQUOR Fructose content of the isomerized liquor was determined by measuring the change in specific rotation which occurred during isomerization. Specific rotations were measured using a Bendix Corporation NPL Model 969 Automatic Polarimeter. The rotations were determined at a concentration of 2.5 g./ 100 ml. in a glass cell thermostated at C. The path of the cell was 50 mm. The specific rotations were determined at the beginning of the isomerization reactions after all ingredients in the glucose-containing solutions had been combined. To determine change in fructose content the specific rotation of the isomerized liquor was determined. All samples were adjusted to pH 4.0 with dilute hydrochloric acid in order to halt enzyme action before dilution for determination of rotations. Change in fructose content was calculated by using the following formula:

A =specific rotation of isomerized liquor. A =specific rotation of glucose-containing solution before isomerization.

In the formula the factor -138.9 is the change in specific rotation which occurs when glucose is converted completely to fructose.

Percent F= IGIU IGIU is the abbreviation of an International Glucose Isomerase Unit and is that amount of enzyme which will convert l micromole of glucose to fructose per minute in a solution containing 2 moles of glucose per liter, 0.02

mole of MgSO per liter, and 0.001 mole of CoCl per liter at a pH of 6.84 to 6.85 (0.2 M sodium maleate) and a temperature of 60 C.

EXTRACTABILITY COEFFICIENT Cellular material containing fixed or stabilized isomerase is held in an aqueous suspension containing 0.001 mole of cobalt chloride per liter at a temperature of 58 C. and a pH of 6.5 (0.05 M sodium maleate buffer). The cellular material is held under these conditions for 20 hours. A portion of the suspension is sonicated at 20 kilocycles by the use of a Branson S75 Sonifier. The sonicated material is centrifuged and the centrifugate analyzed for isomerase activity. Another portion of the suspension (not sonicated) is centrifuged and the isomerase activity of this centrifugate is determined. The activity of extracted isomerase in the latter centrifugate divided by the activity of the isomerase in the centrifugate from the sonioation treatment multiplied by 100 is the extractability coefficient of the treated cellular material.

The process of the present invention provides a number of distinct advantages. For example, it provides a process which can easily and economically be performed in a commercial operation. Furthermore, it is extremely etficient in respect to the utilization of the glucose isomerase and the production of a glucose-fructose syrup containing minimal color, ash and psicose. Moreover, the isomerization reaction may be performed continuously, which, of course, is a distinct advantage in any manufacturing operation.

The characteristics of the glucose-containing solution are somewhat important in determining the exact conditions under which the isomerization reaction is performed. There are many methods known in the art for the production of glucose-containing solutions. For instance, the methods which are currently being practiced commercially principally involve saccharifying corn starch to glucose. These methods may be grouped into three categories. The first is an acid process in which a dilute acid solution is used to hydrolyze starch to glucose. The second is an acid-enzyme process in which starch is liquefied by a mild acid treatment and then an enzyme is used to convert the liquefied starch to glucose. The third is an enzyme-enzyme process in which two enzyme treatments are used, the first to liquefy the starch and the second to convert the liquefied starch to glucose. In the process of the present invention, it is preferred to use a glucosecontaining solution produced by either of the latter two processes since such solutions, generally, contain greater amounts of glucose on a dry substance weight basis, lesser amounts of acids which must be substantially neutralized, less color and lower amounts of oligosaccharides. The glucose-containing solution may be refined, if desired, by conventional means prior to its being subjected to the process of the present invention.

The viscosity of the glucose-containing solution should be in the range of from about 0.5 centipoise to about 100 centipoise and preferably should be from about 2 centipoise to about 20 centipoise. If the viscosity of the solution is too high, the pressure required to pass the solution through the bed will be unreasonably high. Reducing the pressure results in reduction of flow rates so that the time that the solution is in contact with the fixed isomerase may be too long to make effective use of the enzyme. Also, because the length of time that the solution is maintained under isomerization conditions, e.g., temperature, pH, etc., may be excessive, there is the likelihood that an undesirable amount of color and psicose would be produced.

The concentration of glucose in the glucose-containing solution should be in the range of from about 5 to about percent by weight and preferably should be in the range of from about 40 to about 60 percent.

The pH of the glucose-containing solution should be in the range of from about "6 to about 9, preferably from about 6.5 to about 8 and most preferably from about 7 to about 8. It is important to maintain the pH of the glucose-containing solution within this range during the isomerization reaction since if the solution is outside this range the isomerase will be quickly inactivated and/ or large amounts of unwanted by-products such as color bodies and psicose will be produced. In the glucose-containing solution there may be various ion activators and/ or stabilizers for the isomerase, such as soluble salts of cobalt, magnesium, etc.

The characteristics of the bed containing cells of microorganisms which have been treated to inhibit the extraction of the isomerase from the cells are extremely important in respect to the quality of the fructose-glucose solution produced and the commercial utilization of the present process. The bed should contain at least 3 IGIU of glucose isomerase activity per cubic centimeter and preferably contain at least 20 IGIU per cubic centimeter If the bed contains less than 3 IGIU of glucose isomerase activity per cubic centimeter, a greater bed volume may be necessary to isomerize equivalent amounts of glucose. This may present various attendant problems such as greater pressure drops across the bed, longer contact times between the glucose-containing solution and the isomerase to produce the desired degree of conversion of glucose to fructose and greater capital expense because of the greater size of the equipment needed to contain the bed of cells. Furthermore, as the depth of the bed is increased there is a greater tendency for compaction of the bed due to the high pressures which must be employed to pass the glucose-containing solution therethrough. For example, in a relatively shallow bed, the bed can only compact to a small extent while in a bed of greater depth the compaction can be much greater. When this occurs the pressure drop across the bed will increase to such an extent that the pressure necessary to pass the glucose solution through the bed may be extraordinarily high, and may be so high that conventionally constructed equipment can not be used to contain the bed.

The stability value of the fixed glucose isomerase should be at least 50 hours, preferably at least about 300 hours and most preferably at least 400 hours.

The ratio of the depth to the width (or diameter) of the bed of the microorganisms containing cell bound glucose isomerase should be less than 2, preferably in the range of from about 0.01 to about 0.1, and most prefenably in the range of from about 0.02 to about 0.05. It is contemplated that the depth of the bed of the cells containing glucose isomerase will, for instance, be in the range of from a fraction of an inch to about inches. Beds of cells having these dimensions provide the advantage that the pressure drop across the bed is small and that compaction of the bed is minimal. However, since the depth of the bed is relatively small there is a greater tendency of channeling to occur. Channeling results in inefiicient use of the glucose isomerase. If at least two beds and preferably at least six beds, of fixed or stabilized glucose isomerase are positioned in series and there is provision provided for mixing the effluent from a previous bed before it is passed through a subsequent bed, any channeling which might occur would not have a serious effect on the efiiciency of the use of fixed or stabilized glucose isomerase.

Intracellular isomerase may be fixed or stabilized within or on the cells of microorganisms by a variety of treatments. For example, the cells may be subject to a heat treatment while suspended in the original whole fermentor broth or heat treated after being separated from the fermentor broth. Other methods of treating the cells to fix or stabilize the isomerase within the cells are contemplated. The exact me'chan-ism by which these treatments fix or stabilize the isomerase is not precisely known, but it is believed that the enzymes of the cells that cause autolysis of the cells, which results in leakage or extraction of the isomerase, are inactivated by these treatments.

. The conditions of treatment are based upon two criteria. The first is that the treatment conditions must not be so vigorous as to destroy or inactivate too great an amount of glucose isomerase. The second is that the extractability coefficient of the treated cells must be relatively low. Generally, most treatments that fix or stabilize isomerase result in some of the isomerase being inactivated. In respect to the first criterion, the treatment should be carried out such that no more than about 50 percent of the isomerase is inactivated as compared to the 'cells not treated, preferably no more than about 40 percent and most preferably no more than about 15 percent. The extractability coefficient of the treated cells should be less than about 50 percent, preferably less than about 20 percent and most preferably less than about 10 percent.

Glucose isomerase preparations may be divided into two broad categories, depending upon whether they are heat stable or heat labile. Exemplary of heat labile isom-erase preparations are those derived from Pseudomonas hydrophila and certain other microorganisms. In order for these cell preparations to convert any appreciable amount of glucose to fructose there generally must be present during the isomerization reaction, arsenate or fluoride ions. The heat stable isomerase preparations do not require that such ions be present during isomerization to convert appreciable quantities of glucose to fructose. Examplary of microorganisms which produce heat stable glucose isomerase preparations are the microorganisms belonging to the Streptomyces genus. In the present process it is preferred to use heat stable isomerase preparations. When such are used, high temperatures may be employed to inhibit or prevent microbial contamination of the bed. Another benefit derived from using the heat stable isomerase preparations is that a convenient heat treatment may be used to fix the isomerase.

The heat treatment may comprise heating cells containing heat stable isomerase to a certain temperature and maintaining the cells at this temperature for a certain period of time. The temperature and the time of treatment are interdependent. At high temperatures only short times are needed While at lower temperatures longer times are required. The heat treatment should be carried out under conditions such that no more than about 50 pencent of the isomerase is destroyed by the treatment, preferably no more than about 40 percent and most preferably no more than about 15 percent. Generally, when cells containing heat stable isomerase are heated at a temperature, in the range of from about 60 to about 90 C. for a time of from about 60 minutes to about 1 minute' no more than 40 percent of the isomerase is destroyed. The pH of the medium in which the cells are treated is preferably in the range of from about 6 to about 9. Optimum results have been obtained in the temperature range of from about 70 to about C. for a time of from about 10 to about 20 minutes and at a pH in the range of from about 7 to about 8.

An apparatus known in the art which can be used to carry out the present process is a pressure leaf filter. The pressure leaf filter comprises an assembly of flat filtering elements (leaves) supported vertically or horizontally in a cylindrical tank. The leaves may be circular or rectangular and have filtering surfaces on both sides. The cylindrical tank may have its long axis horizontal or vertical. A filter leaf may consist of a heavy screen or grooved plate over which a filter medium such as woven fabric or fine wire cloth is fitted. The cellular material containing fixed isomerase may be slurried in a glucosecontaining solution and this slurry pumped through the pressure leaf filter in such a manner as to evenly cover each leaf with the cells. The pressure applied to the solution will hold, in the case of a vertical pressure leaf filter, the cells to the leaves. A glucose-containing solution may then be pumped through the pressure leaf filter and while it passes through each bed of cellular material containing fixed isomerase, isomerization will occur. The amount of fructose formed will be dependent upon the period of time that the glucose solution is in contact with the fixed enzyme.

The exact composition of the isomerized glucose-containing solution will vary depending upon the exact conditions under which the present process is performed. In Table I, below, the essential characteristics of isomerized glucose-containing solutions produced by the present invention are shown.

TABLE I Characteristics of Isomerized Glucose Containing Solutions Percent dry basis Fruc- Polysac- Color Ranges Glucose tose charides Psicose Ash 2 units Typical 30 to 60 10 to 54 to 50 0 to 1.0 0.1 to 0. 0 to 2 Preferred. 30 to 60 to 54 0 to 30 0 to 0. 5 0.1 to 0.2 0 to 0.05 Most preferre 30 to G0 10 to 54 0 to 30 0 to 0. 1 0.05 to 0. 1 0 to 0. 03

1 Amount of the saccharides principally dependent upon the characteristics desired in the product.

2 Principally composed of metallic salts which are present during isomerization to stabilize and/or activate the glucose isomerase.

In order to describe more clearly the nature of the solution having aviscosity of from about 0.5 to about 100 present invention, a specific example will hereinafter be described. It should be understood, however, that this is done solely by way of example and is intended neither to delineate the scope of the invention nor limit the ambit of the appened claims.

EXAMPLE This example illustrates the use of fixed or stabilized glucose isomerase to continuously convert glucose to fructose.

Streptomyces ATCC 21175 was grown under submerged aerobic conditions. The pH of the broth containing the cellular material was adjusted to 7.5, and the broth heated for one-half hour until it reached 75 C. and was maintained at this temperature for 5 minutes to fix the isomerase to the cells. Three percent by weight of filter aid was incorporated into the broth and then the broth was filtered on a rotary drum vacuum filter precoated with diatomaceous earth. The cellular material was washed on the filter. The resulting filter cake was dried in a forced air drier at an air temperature of 49 C. for about 3.5 hours to yield a dried, fixed isomerase preparation compisin-g the cellular material containing the fixed isomerase admixed with filter aid. The extractability coefficient of the dried, fixed isomerase preparation was 15 percent and the stability value was 645 hours.

Two hundred fifty-six (256) pounds of the dried, fixed isomerase preparation was blended for about 2 minutes and was then slurried with a glucose-containing solution consisting of a refined corn starch hydrolysate at a pH of 7, containing 60 g. of glucose per 100 ml., 4.5 g. of maltose, isomaltose and oligosaccharides per 100 ml., 0.001 moles of CoCl per liter, 0.01 mole of MgSO per liter and 0.006 mole of NaHSO per liter. The temperature of the glucose solution was 65 C. The slurry was pumped through a pressure leaf filter (manufactured by Industrial Filter Pump Mfg. Co., Cicero, Ill.) having 6 filter leaves and each leaf was coated with a 1 to 1.5 inch layer of the fixed isomerase preparation. The total activity of the isomerase on the leaves was 1.26 10 IGIU. The filter leaves had a total surface area of 74 ft. The glucose solution was pumped continuousy through the filter at a rate such that 45 percent of the glucose was converted to fructose. The flow rate was gradually reduced as the activity of the fixed isomerase on the leaves diminished, so that the percent conversion of glucose to fructose was maintained constant at 45 percent. At the end of about 40 hours, the flow rate through the leaves was 1.0 gallon per minute. At the end of 200 hours the flow rate was 0.5 gallon per minute and the centipoise, a pH in the range of from about 6 to about 9 and containing from about 5 to about percent glucose by weight; heating said solution to a temperature of from about 20 to about 80 C., and passing said solution through a bed containing cells of microorganisms containing intracellular glucose isomerase which have been treated to inhibit the extraction of the isomerase from the cells having a glucose isomerase activity of at least about 3 IGIU per cubic centimeter of bed, a stability value of at least about 50 hours and a depth to width ratio of less than about 2 at a flow rate whereby up to 54 percent of the glucose by weight is converted to fructose, the color of the converted solution is increased by less than 2 color units and there is no substantial production of psicose.

2. A process of enzymatically converting glucose to fructose as defined in claim 1, wherein the viscosity of the glucose-containing solution is from about 2 to about 20 centipoise and the pH of the solution is from about 6.5 to about 8.

3. A process of enzymatically converting glucose to fructose as defined in claim 2, wherein the glucose-containing solution contains from about 40 to about 60 percent glucose by weight and the pH of the solution is from about 7 to about 8.

4. A process of enzymatically converting glucose to fructose as defined in claim 2, wherein the bed containing cells of the microorganisms contains at least 20 IGIU per cubic centimeter of bed.

5. A process of enzymatically converting glucose to fructose as defined in claim 4, wherein the stability value of the cells of the microorganisms is at least 300 hours.

6. A process of enzymatically converting glucose to fructose as defined in claim 5, wherein the stability value of the cells of the microorganisms is at least 400 hours.

7. A process of enzymatically converting glucose to fructose as defined in claim 5, wherein the bed has a depth to width ratio in the range of from about 0.01 to about 0.1.

8. A process of enzymatically converting glucose to fructose as defined in claim 7, wherein the bed has a depth toowidth ratio in the range of from about 0.02. to about 0. 5.

9. A process of enzymatically converting glucose to fructose as defined in claim 7, wherein the cells are derived from microorganisms belonging to the Streptomyces genus.

10. A process of enzymatically converting glucose to fructose as defined in claim 9, wherein the glucose-containing solution is passed through at least 2 beds which are positioned in series.

11. A process of enzymatically converting glucose to fructose as defined in claim 1, wherein the cells of the microorganism are derived from Streptomyces ATCC 21175.

12. A process of enzymatically converting glucose to fructose as defined in claim 1, wherein the gluctose-containing solution passed through the bed is produced by treating starch with an acid to liquefy the starch and then treating the liquefied starch with a glucose forming enzyme to obtain the glucose-containing solution.

13. A process for enzymatically converting glucose to fructose as defined in claim 1, wherein the glucose-containing solution passed through the bed is produced by treating starch with an enzyme to liquefy the starch and References Cited UNITED STATES PATENTS 2/1933 Currie et al. 195--47 OTHER REFERENCES Takasaki et al.: Ferm Adv. by Perlman, pp. 561- 589, Acad. Press, 1969.

A. LOUIS MONACELL, Primary Examiner then treating the liquefied starch with a glucose forming 15 G. M. NATH, Assistant Examiner enzyme to obtain the glucose-containing solution. 

